There should be transfer passengers. That arrives on one flight and then spend some time in the airport. Later they board another flight. Either 2 hour later or 8 hours later depending on destination and airline and planned flights.
Why implement this.
For more realism and verity among passengers.
This also means not all passengers have to leave or enter the airport via check-in and in extent via public transport.
Practically from the top of my head this will be the game criteria for spawning a transfer passenger.
*Two or more flights is planned a few hours in advance, on the same day.
*Minimum transfer time is 2 hours.
(If there is no flight with 2 hours separation after arrival flight, and on the same day. The passenger will not spawn)
*Departing flight can not go to the destination that the passenger arrived on.
In the long run.
We could have passengers that arrive on Monday and leave Thursday. They have then stayed at hotel for their visit.
Also we should be able to see how many times a passenger has visit our airport.